Ladder-type filters have been used in high frequency communication systems. It has been known to connect an inductor between a parallel resonator of a ladder-type filter and ground as disclosed in Japanese Patent Application Publication No. 2002-314372 (Patent Document 1). It has been also known to connect a capacitor between the ground terminal and the serial arm of a parallel resonator as disclosed in Japanese Patent Application Publication No. 2014-17537 (Patent Document 2).
The arts disclosed in Patent Documents 1 and 2 allow an attenuation pole to be formed outside the passband of the ladder-type filter. When an attenuation pole is formed, the loss of the passband and the characteristics including an isolation characteristic are required not to deteriorate before and after the connection of an inductor. For example, if the resonant frequency of a resonator changes due to the addition of the inductor, the characteristics of the ladder-type filter changes.